1. Field of the Invention
The present invention relates to techniques for forming semiconductor thin films to be used for a semiconductor device such as a thin film transistor. Particularly, the present invention relates to a technique of forming a semiconductor film by light irradiation. For example, the present invention is applicable to a technique of obtaining a field-effect transistor by forming a gate insulating layer on a single-crystal silicon film, and a driving circuit for a display, a sensor, etc. that are structured by these semiconductor thin films and field-effect thin film transistors.
2. Description of the Related Art
First, state-of-the-art techniques in the above-described technical field will be explained in detail, and problems will be pointed out.
As representative techniques of forming a thin-film transistor (TFT) on a glass substrate, there have been known a hydrogenated amorphous silicon (a-Si:H) TFT technique and a poly-crystal silicon (poly-Si) TFT technique. The a-Si:H TFT technique has a maximum temperature of about 300° C. in a manufacturing process, and achieves a carrier mobility of about 1 cm2/Vsec. This technique is used as a switching transistor for each pixel In an active matrix type (AM-) liquid crystal display (LCD). This switching transistor is driven by a driver integrated circuit (an LSI formed on a single-crystal silicon substrate) disposed on the periphery of a screen area. Since a switching TFT is provided for each pixel, this has a feature that it is possible to obtain a satisfactory picture quality by reducing cross-talks, as compared with a passive matrix type LCD that sends an electric signal for driving a liquid crystal from a separate peripheral driver circuit.
On the other hand, the poly-Si TFT technique uses a high-temperature process similar to LSI process of about 1,000° C. by using, for example, a quartz substrate. Thus, it is possible to obtain a carrier mobility of 30 to 100 cm2/Vsec. Since such a high carrier mobility can be realized, when this is applied to a liquid crystal display, for example, it is possible to simultaneously form a pixel TFT for driving each pixel and a peripheral driving circuit on the same substrate. Therefore, there is an advantage that it is possible to contribute to a reduction in the manufacturing process cost and to reduce the size of a device. Along with a reduction in the size of a device and an introduction of a high resolution, a bonding pitch between an AM-LCD substrate and peripheral driver integrated circuits must be made smaller. A tape automated bonding or a wire bonding method cannot cope with this situation.
However, a low-cost low-softening-point glass that can be used for the a-Si:H TFT technique cannot be used in the poly-crystal silicon TFT technique because of the above-described high-temperature process. For improving this point, it is necessary to lower the temperature in the poly-crystal silicon TFT technique. In order to meet this requirement, there has been progressed a research and development of a technique of forming a poly-crystal silicon film at low temperature.
As has been known, generally, the laser crystallization is achieved by using a pulse laser beam irradiator.
FIG. 1 is a schematic view showing one example of a structure of a pulse laser beam irradiator. A laser beam (1106) supplied from a pulse laser beam source (1101) reaches a silicon thin film (1107) on a glass substrate (118) as an irradiated object through an optical path defined by optical devices such as mirrors (1102, 1103, 1105) and a beam homogenizer (1104) installed for homogenizing a spatial intensity. Generally, since one irradiation range is small, a laser beam irradiation is carried out at a desired position on the glass substrate by moving the substrate on an x-y stage (1109). There is also a method of moving the optical devices or combining the optical devices with the stage instead of moving the x-y stage.
For example, an example of mounting a substrate on an x-direction stage and mounting a homogenizer on a y-direction stage is shown in a paper written by J. Im and R. Sposili, Crystalline Si films for integrated active-matrix-liquid-crystal displays. Materials Research Society Bulletin magazine. vol. 21, (1996), 39 (hereafter, referred to as publication 1).
A laser beam irradiation is also carried out in vacuum or in a high-purity gas atmosphere within a vacuum chamber. The publication 1 also describes that, as demanded, a cassette (1110) storing a glass substrate on which a silicon thin film is formed and a substrate carrying mechanism (1111) are also provided to mechanically take out the substrate from the cassette to the stage and accommodate the substrate in the cassette.
Further, there is also disclosed a technique of crystallizing an amorphous silicon thin film on an amorphous substrate by Irradiating a short-wavelength pulse laser beam and applying this to a thin-film transistor in Japanese Patent Application Examined Publication No. 7-118443. According to the disclosed method, it is possible to crystallize amorphous silicon without heating the whole substrate to a high temperature. Therefore, there is an advantage that it is possible to manufacture a semiconductor device or a semiconductor integrated circuit on a large area like a liquid crystal display or on a low-cost substrate like glass.
However, as described in the publication, an irradiation intensity of about 50-500 mJ/cm2 is necessary for crystallizing an amorphous silicon thin film by using a short-wavelength laser beam. On the other hand, a light-emission output of a pulse laser device generally obtainable at present is about 1 J/pulse at maximum. Therefore, based on a simple conversion, an area that can be processed by irradiation at one time is only about 2 to 20 cm2. Therefore, in order to crystallize by laser beams the whole surface of a substrate having a substrate size of 47×37 cm, for example, it is necessary to irradiate laser beams to at least 87 positions, or 870 positions depending on a condition. When the substrate has a large size like a substrate size of 1 by 1 meters square, the number of positions where irradiation is necessary increases accordingly. The laser crystallization in this case is also carried out by using a pulse laser beam irradiator of the structure shown in FIG. 1.
In order to form a plurality of thin film semiconductors homogeneously on a large-area substrate by applying the above method, it has been known that a technique as disclosed in Japanese Patent Application Unexamined Publication No. 3-315863, for example, is effective. That is, a method of dividing semiconductors into areas, each being smaller than a beam size, and based on a step-end-repeat manner, sequentially repeating an irradiation of a few pulses + a move of an irradiation area + an irradiation of a few pulses + a move of an irradiation area + - - -, is effective. There is also a control method that a laser beam oscillation and a move of a stage (that is, a substrate) or a beam are carried out alternately, as shown by a laser operation method in FIG. 2A.
However, according to this method, even when a currently available pulse laser device having oscillation intensity homogeneity of ±5 to 10% (during a continuous oscillation) is used, there is the following problem. For example, when an irradiation of 1 pulse to 20 pulses/position is repeated, an oscillation intensity variance exceeds ±5 to 10%. As a result, the homogeneity of a poly-crystal silicon thin film and poly-crystal silicon thin film transistor characteristics obtained Is not sufficient. Particularly, an occurrence of a strong beam or a weak beam caused by an unstable discharging at an initial stage of a laser beam oscillation, which is called a spiking, is one of factors of inhomogeneity.
In order to correct the above difficulty, a method of controlling an applied voltage at the time of a next oscillation based on a result of intensity accumulation has been also known. However, this has a problem in that a weak beam is oscillated although it is possible to restrict an occurrence of spiking.
As shown in FIG. 3, when an irradiation time and a non-irradiation time continue alternately, the intensity of a first pulse oscillated during each irradiation time becomes most unstable and varies. Further, an irradiation intensity hysteresis is different depending on an irradiation position. Therefore, there arises a problem that it is not possible to obtain sufficient homogeneity of transistors and thin film integrated circuits on the substrate.
As another method of avoiding such a spiking, there has been known a method of avoiding the spiking by starting a laser beam oscillation before starting an irradiation in the device forming area, as shown in FIG. 2B. However, this method cannot be applied when a laser beam oscillation and a stage move as shown in FIG. 2A are repeated intermittently.
In order to avoid these problems, Japanese Patent Application Unexamined Publication No. 5-90191 has proposed a method of continuously oscillating beams from a pulse laser beam source and not irradiating to a substrate during a stage move period by using a light shielding device. In other words, as shown in FIG. 2C, laser beams are oscillated continuously in a certain frequency, and a move of a stage to a desired irradiation position and a shielding and a release of an optical path are synchronized. This makes it possible to irradiate a desired irradiation position with a laser beam of a stable intensity. According to this method, it becomes possible to irradiate stable laser beams to the substrate. However, this method has a problem in that a wasteful laser beam oscillation increases that does not contribute to the formation of a poly-crystal silicon thin film. Further, productivity of a poly-crystal silicon thin film to the high-cost laser beam source and the life of an excitation gas are lowered, and productivity of a poly-crystal silicon thin film to electric power required for a laser beam oscillation is lowered. Therefore, the production cost increases.
Further, when a substrate has been irradiated with a laser beam of an excessive intensity as compared with the desired intensity due to a variance of the irradiation intensity, there is a risk of an occurrence of damage to the substrate. Particularly, after a laser beam has been transmitted through a substrate in an imaging device like an LCD, this incurs a light scattering in a damaged area of the substrate, which results in a reduction in the picture quality.
Next, a technique of compressed-projecting a pattern on an optical mask onto a silicon thin film and crystallizing it according to the pattern is disclosed, for example, in a paper written by R. Sposili and J. Im, “Sequential lateral solidification of thin silicon films on SiO2”, Applied Physics Letters, vol. 69, (1996), 2864 (a publication 2), and a paper written by J. Im, R. Sposili and M. Crowder, “Single-crystal Si films for thin film transistor devices”, Applied Physics Letters, vol. 70, (1997), 3434 (a publication 3).
According to each of the above publications 2 and 3, a compressed projection of about 1:5 is carried out by using 308 nm excimer laser, a variable-energy attenuater, a variable-focus field lens, a patterned-mask, a two-clement imaging lens, and a sub-micrometer-precision translation stage. Thus, a beam size of μm order and a moving pitch of a substrate stage of μm order are achieved.
However, when this method is used for processing a large substrate as described above, a laser beam with which the optical mask is irradiated has a spatial intensity profile depending on the light source. Therefore, a critical intensity distribution deviation occurs in an exposure pattern transmitted through the center and the periphery of the mask, for example. This has made it impossible to obtain a crystalline silicon thin film having a desired homogeneity. Further, since an ultraviolet ray having a short wavelength is compressed-projected, the depth of focus is small, which has a problem of generating an irradiation depth deviation due to a camber and a flexure. Further, as the substrate becomes larger, it becomes difficult to secure a mechanical precision of the stage. Furthermore, a declining of the stage and a deviation of the substrate on the stage during a move adversely affect the desired irradiation condition of a laser beam.
In carrying out the above laser beam irradiation, a method of emitting a plurality of pulses with a certain delay time has been disclosed in a paper; Ryoichi Ishihara et al., “Effects of light pulse duration on eximer laser crystallization characteristics of silicon thin films”, Japanese journal of applied physics, vol. 34, No. 4A, (1995) pp 1759 (a publication 4). According to this publication 4, the crystallization solidification rate of a molten silicon in a laser recrystallization process is 1 m/sec or above. In order to obtain a satisfactory crystallization growth, it is necessary to lower the solidification rate. When a second laser beam pulse is emitted immediately after a completion of the solidification, a recrystallization process with a small solidification rate is obtained by the second irradiation.
According to a temperature change (a time-varying temperature curve) of silicon as shown in FIG. 4, the temperature of silicon rises along with the irradiation of laser energy (for example, an intensity pulse showing a waveform in FIG. 5). When a starting material is a-Si, the temperature further rises after passing a fusing point of a-Si. When the supply of energy becomes lower than a value necessary for a temperature rise, cooling starts. At a solidification point of the crystalline Si, solidification is carried out and then the solidification finishes. After that, the crystalline Si is cooled to an atmospheric temperature. When the solidification of the silicon proceeds in a film-thickness direction starting from a silicon-substrate interface, the average value of the solidification rate is expressed by the following expression:Average value of solidification rate=film thickness of silicon/solidification time. 
In other words, when the silicon film thickness is constant, it is necessary to make longer the solidification time so as to make the solidification rate slower. Accordingly, in a process of maintaining an ideal state in terms of thermal equilibrium, it is possible to expand the solidification time by increasing an ideal input energy, that is, laser beam irradiation energy.
However, as pointed out in the above publication, there is a problem that an increase in the irradiation energy causes the silicon film to be amorphous or of fine crystallization. In the real fusion and recrystallization process, the ideal temperature change as shown in FIG. 4 is not obtained. Generally, the temperature is raised excessively during a heating time, and the temperature is lowered excessively during a cooling, and then a stabilized state reaches. Particularly, when the cooling rate during the cooling period is large and a process of excessive cooling is experienced, crystallization does not occur near the solidification point. Instead, an amorphous solid is formed by a rapidly cooled solidification. In the case of a thin film, instead of forming an amorphous film, a fine crystallized film is formed depending on the condition, as described in the above publication. The fine crystal film has extremely small particles as compared with a poly-crystal thin film or a single-crystal thin film. Therefore, this has a problem in that there exist a large number of crystalline grain boundaries of a large grain boundary potential, and an ON current is lowered or an off-leak current increases when this method has been applied to a thin-film transistor, for example.
FIG. 6 shows a maximum cooling rate (Cooling rate, K/sec) obtained by numerical computation when an excimer laser beam of a wavelength 308 nm has been emitted onto a silicon thin film of a film thickness 75 nm, and a threshold value of irradiation intensity for crystallization to fine crystallization obtained from an SEM observation of the film after the laser beam irradiation. A pulse waveform of a laser beam emission used for the experiment is shown in FIG. 5. This has three main peaks, and the light emission time is about 120 nsec. This pulse waveform has a light emission time of more than five times that of a square pulse of a pulse width 21.4 nsec that is assumed and described in the publication 4. Therefore, it can be expected that a reduction in the solidification rate as described in the publication 4 can be achieved in a single pulse irradiation.
A temperature-time curve of silicon obtained by numerical computation at the time of a laser recrystallization using the above pulse waveform becomes as shown in FIG. 7.
FIG. 7 shows a temperature change of a silicon thin film when the silicon thin film has a film thickness of 75 nm, a substrate used is SiO2, and irradiation intensity of XeCl laser beam (a wavelength of 308 nm) is 450 mJ/cm2. The temperature reached a maximum temperature in about 60 nsec after a peak of a second light emission had almost finished, and then the cooling is started. (In the present numerical computation, the fusion and solidification point values of amorphous silicon is used in this case. A behavior near the solidification point is different from a real one. Particularly, when a crystallized film is obtained, the crystallization is completed near the solidification point of the crystalline silicon).
It can be known that the cooling is started by once having a large tilt, and the tilt of about 100 nsec when a third peak exists becomes very small. After a lapse of 120 nsec when the light emission finishes completely, a rapid cooling occurs again, and then solidification occurs. Generally, in the case of a process of solidification from a liquid that has been subjected to “rapid cooling” that greatly deviates from a thermal equilibrium process, it is not possible to obtain sufficient solidification time necessary for forming a crystal structure, and an amorphous solid is formed.
FIG. 6 shows an estimated result of a maximum cooling rate after finishing a light emission at each irradiation intensity based on the temperature-time curve of silicon shown in FIG. 7. It can be known that the cooling rate increases along with an increase in the irradiation intensity. On the other hand, a structure of a silicon thin film after the laser beam irradiation has been observed with a scanning-type electronic microscope. As shown in FIG. 6, a particle size once increases along with an increase in the irradiation intensity. However, a fine crystallization has been observed under a set irradiation intensity condition of about 470 mJ/cm2. Similarly, when the number of irradiation pulses has been set to three pulses, a partially fine crystallized area remains under the set irradiation intensity condition of about 470 mJ/cm2. However, unlike the case of one pulse, a remarkable increase in the particle size has been observed (see FIG. 8 showing a crystal state of a laser recrystallized silicon thin film corresponding to each irradiation intensity and number of irradiation).
Actual irradiation intensity becomes about 5 to 10% higher than a set value particularly in a first few pulses of an excimer laser beam. Therefore, a threshold intensity at which a fine crystallization occurs can be estimated as about 500 mJ/cm2. From the above result, by assuming a cooling rate based on the condition of 500 mJ/cm2 shown in FIG. 6, it has been known that a fine crystallization occurs under the cooling rate condition of about 1.6×1010° C./sec or above. When an irradiated film is a-Si, a fine crystallization occurs under the condition of about 500 mJ/cm2 or above. Similarly, when an irradiated film is poly-Si, by applying this cooling rate, an irradiation intensity that is about 30 mJ/cm2 larger than that of a-Si is indicated. Therefore, by controlling the cooling rate to 1.6×1010° C./sec or lower, it becomes possible to prevent a fine crystallization and an amorphous state. As a result, it is possible to obtain a satisfactory crystal growing process.
Next, an introduction of a second laser beam by delaying it after a first laser beam will be explained. As already described, a laser beam at a later period of light emission mitigates an increase in the cooling rate, and the cooling rate after finishing the light emission rules the crystallization. In other words, it is considered that the preceding cooling process is initialized by energy finally introduced. When further additional energy is introduced, the process is considered to be once initialized and solidification is repeated again even if an amorphous state and fine crystallization has occurred due to rapid cooling in the preceding solidification process, because energy has been stored. (Because of a short period of time like a nanosecond order, thermal conduction to the substrate and irradiation to the atmosphere is considered to be small. Of course, the time during which a sufficient heat discharging is possible is not taken into consideration.) Therefore, it is possible to expect a satisfactory crystal growth by a rapid cooling rate after finishing a secondary heating by the energy input again. As a maximum cooling rate and a cooling rate near a solidification point show in FIG. 9, the cooling rate is controlled to a desired value by controlling the delay time.
On the other hand, a technique of carrying out a process of forming an a-Si thin film as a laser beam irradiated material, a laser beam irradiation process, a plasma hydrogenation process and a process of forming a gate insulation film, sequentially or by changing the sequence, without an exposure to the atmosphere, has been disclosed in the following publications.                Japanese Patent Application Unexamined Publication No. 5-182923; after thermally processing an amorphous semiconductor thin film, a laser beam is irradiated without an exposure to the atmosphere.        Japanese Patent Application Unexamined Publication No. 7-99321; a substrate having a laser crystallized poly-crystal silicon thin film Is carried to a plasma hydrogenation and gate insulation film forming processes without exposing the substrate to the atmosphere.        Japanese Patent Application Unexamined Publication No. 9-7911; a substrate having a laser crystallized poly-crystal silicon thin film is carried to a gate insulation film forming processes without exposing the substrate to the atmosphere.        Japanese Patent Application Unexamined Publication No. 9-17729; a substrate having a laser crystallized poly-crystal silicon thin film is carried to a gate insulation film forming processes without exposing the substrate to the atmosphere, thereby preventing an adhesion of an impurity to the surface of the poly-crystal silicon.        Japanese Patent Application Unexamined Publication No. 9-148246; a formation of an amorphous silicon thin film, a laser crystallization, a hydrogenation, and a formation of a gate insulation film are carried out continuously without an exposure to the atmosphere.        Japanese Patent Application Unexamined Publication No. 10-116989; a formation of an amorphous silicon thin film, a laser crystallization, a hydrogenation, and a formation of a gate insulation film are carried out continuously without an exposure to the atmosphere.        Japanese Patent Application Unexamined Publication No. 10-149984; a formation of an amorphous silicon thin film, a laser crystallization, a hydrogenation, and a formation of a gate insulation film are carried out continuously without an exposure to the atmosphere.        Japanese Patent Application Unexamined Publication No. 11-17185; a formation of an amorphous silicon thin film, a laser crystallization, a formation of a gate insulation film, and a formation of a gate electrode are carried out continuously without an exposure to the atmosphere.        
The ideas and techniques shown in these publications have been devised so as to solve a problem that, as the surface of silicon formed by a laser crystallization is very active, impurities are easily adhered to the silicon surface when the surface is exposed to the atmosphere, and as a result, characteristics of a TFT are deteriorated or the characteristics are varied.
In order to evaluate the above-described techniques, the applicants have implemented an excimer laser crystallization technique and a silicon oxide thin-film forming technique on the same device (Including conveyance of a substrate to another device without an exposure to the atmosphere), and have compared the performance of a finished product with that manufactured by involving an exposure to the atmosphere. As a result, a large effect has been confirmed in the improvement of a product yield due to the effect of preventing an adhesion of dusts and particles. On the other hand, it has been confirmed that it is also possible to obtain a similar effect to that obtained by increasing the clean level of a clean room environment.
For improving the product yield, there is obtained a largest effect when a substrate cleaning mechanism is built into the same device compared with an effect obtained from the film-forming device. For example, during a film-forming process, particles may be adhered to the surface of the substrate according to a condition of forming a-Si. This required the a-Si to be once released to the atmosphere to have a cleaning process. On the other hand, looking at the performance of a thin-film transistor, there has been no remarkable difference between the manufacturing processes. The reason is considered as follows.
The present applicants have disclosed a fixed oxide film charge density (1011 to 1012 cm−2) of a silicon oxide film that is formed by using a plasma at temperatures around 300 to 350° C. and a silicon oxide film formed by a thermal treatment of around 600° C., and an interface state density (up to 6×1010 cm−2 eV−2) between a silicon oxide film and a silicon substrate in the following paper, for example; K. Yuda et al., “Improvement of structural and electrical properties in low-temperature gate-oxides for poly-Si TFTs by controlling O2/SiH4 ratios”, Digest of technical papers, 1997 international workshop on active matrix liquid crystal displays, Sep. 11-12, 1997, Kogakuin Univ., Tokyo, Japan, 87 (a publication 5). In this case, a silicon substrate Is generally cleaned by what is called an RCA cleaning using an acid (hated according to the need) cleaning liquid such as sulfuric acid/hydrogen peroxide water, hydrochloric acid/hydrogen peroxide water/water, ammonium/ hydrogen peroxide water/water, hydrofluoric acid/water. The silicon substrate is then washed and is introduced into a film-forming device. Therefore, the interface state density value is obtained from a sample of a single-crystal silicon substrate that has been clean-interface formed (cleaned) and then exposed to the atmosphere and shifted to a film-forming process.
A trap state density which is the other density of a laser single-crystal silicon film will be focused. The present applicants have disclosed a trap state density (1012 to 1013 cm−2) in a crystallized silicon film from a thin-film transistor having a laser crystallized silicon film in, for example, a paper written by H. Tanaka et al., “Excimer laser crystallization of amorphous silicon films”, NEC Resource and Development magazine, vol. 35. (1994), 254, (a publication 6). The field-effect mobility of these transistors shows satisfactory characteristics like 40 to 140 cm2/Vsec.
When the trap state density of the silicon film is compared with the interface state density (or a fixed oxide film charge density), it is clear that the trap state density value is larger. In other words, it has been made clear that in order to obtain the effect of cleaning in the sample obtained by forming a silicon film/gate insulation film within the same device without an exposure to the atmosphere, the performance (trap state density) of the silicon film is not sufficient.
As a means for forming a satisfactory gate insulation film by reducing plasma damage in the field relating to the present invention, there has been proposed a remote plasma CVD (chemical vapor deposition) method. For example, a structure having a plasma generating chamber and a substrate processing chamber disposed separately is disclosed in Japanese Patent Application Unexamined Publication No. 5-21393. It is considered possible to achieve the above-described low fixed oxide film charge density (1011 to 1012 cm−2) and the low interface state density (up to 6×1010 cm−2 eV−2). However, as described above, there has been a problem that this effect is limited by the performance of the silicon film formed in advance.